A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
European Patent Application No. EP-A-0 957 402 describes that a lithographic apparatus may suffer from the problem that gas phase contaminants may cause deterioration of optical elements (mirror lenses and/or transmissive lenses) that are used to image the pattern onto the layer of radiation-sensitive material on the substrate. The resist material in particular is a source of light molecules that evaporate from the resist during irradiation of the substrate. This may be a particular problem when (E)UV irradiation is used to transfer the pattern, but to a lesser extent, similar problems may arise also when longer irradiation wavelengths are used.
EP-A-0 957 402 discloses a solution to this problem. A tube is provided between an optics compartment of the lithographic apparatus that contains optical elements, and a substrate compartment that contains the substrate. The imaging radiation passes through this tube. A gas flow also passes through this tube to carry away (flush) the contaminants from the substrate compartment before they reach the optic compartment. Typically, a gas inlet is provided on one side of the tube and a pump is coupled to another side of the tube. Gases, such as Ar or Kr, that are relatively transparent for the imaging radiation are used. As EP-A-0 957 402 points out, these gases have a low diffusion coefficient for contaminants, that is, the majority of the contaminants will not be able to reach the optics compartment before they are carried away by the gas flow.
Nevertheless, it has been found that a considerable gas flow strength may be needed to ensure that a satisfactory amount of contaminants is carried away. This may require heavy pumping, which means that considerable overhead is may be needed to prevent vibrations from the pumps from affecting the imaging process. The heavy pumping may complicate the design of the lithographic apparatus and its maintenance. Also, the high flow strength may require relatively high pressure, which may have drawbacks, such as increased absorption, secondary emission, and damage to the optical elements.
U.S. Pat. No. 6,614,505 describes the problem of contaminants that emerge from the radiation source that is used to generate the radiation with which the pattern is imaged onto the substrate. A plasma source may be used for example. U.S. Pat. No. 6,614,505 describes several approaches to remove these contaminants before they flow from the radiation source to the optics compartment of the lithographic apparatus. U.S. Pat. No. 6,614,505 mentions that these solutions may also be used between the substrate compartment and the optics compartment.
U.S. Pat. No. 6,614,505 proposes the use of charged getter electrodes to attract contaminants before they can reach the optical elements. U.S. Pat. No. 6,614,505 is primarily concerned with contaminants that evaporate as neutral particles from the radiation source. U.S. Pat. No. 6,614,505 discloses how these particles are charged to ensure that they are attracted by the getter electrodes. U.S. Pat. No. 6,614,505 discloses several methods of charging the particles. According to a first method, an electron source is provided adjacent a tube that connects the compartment where the radiation source is located and the optics compartment. The electron source emits electrons that ionize the contaminant particles in the tube so that they will be attracted by the getter electrodes. According to a second method, a plasma (a gas containing ions and free electrons) is maintained in the tube by applying an RF electromagnetic field to noble gas that is present in the tube. The plasma ionizes the contaminants which are subsequently attracted to the charged walls of the tube, which serve as getter electrodes in this case.